Drill string

ABSTRACT

A drill string for use in the directional drilling of wellbores is described. A downhole reamer apparatus (202) is mounted in a drill string incorporating a drilling motor (300), stabiliser (302) and drill bit (304) operable either by rotation of the drill string or activation of the drilling motor (300). The drill string is located in wellbore (306) at the end of drill pipe (310). If the operator wishes to drill upwardly, the reamer elements are locked in the retracted condition. This results in the reamer apparatus being under gauge which results in build angle (308) pointing the drill bit upwardly.

The present invention relates to a drill string for use in the directional drilling of wellbores and relates particularly, but not exclusively, to a drill string comprising a downhole reamer apparatus for use to in both ream while drilling in directional slide drilling operations. The present invention also relates to a downhole reamer apparatus for use to in both ream while drilling in directional slide drilling operations.

In conventional directional drilling to form wellbores for use in hydrocarbon production, the process of slide drilling is used generally from 10% to as high as 30% of the time during the drilling operation. Slide drilling is used to make a directional correction or change the drilling angle.

In order to do this, rotation of the drill string from the surface is stopped by locking the rotary table or rotary top drive. A drilling motor (generally known as a mud motor), fitted with a bent housing or bent sub, is actuated by pumping drilling mud through the drill string. The drill bit is then pointed in the desired direction and the drill string pushed or slid to advance along the wellbore.

If the operator wishes to enlarge the diameter of the drilled hole, ream while drilling tools are used. However, the reaming elements of the reamer create drag when the drill string is moved along the wellbore in slide mode. In some circumstances, the drill string will not slide when the well bore enlargement tool cutters reach the undersized pilot hole drilled by the bit. The drill string weight will be held on the cutters resting on the ledge at the well bore transition.

The present invention seeks to overcome the above disadvantages of the prior art.

According to an aspect of the present invention, there is provided a drill string comprising:

-   -   a drill bit;     -   a drilling motor, wherein the drill bit is configured to be         rotated either by rotation of the drill string or by operation         of the drilling motor; and     -   a downhole reamer apparatus comprising:         -   a mandrel arranged to be disposed in the drill string;         -   an outer housing disposed on the mandrel, wherein the             mandrel is rotatable relative to the outer housing; and         -   at least one reaming element moveable between an inwardly             retracted condition and an outwardly deployed condition in             response to rotation of the mandrel in a first direction             relative to the outer housing;     -   wherein operation of the drilling motor to rotate the drill bit         causes a reaction torque in the drill string in a second         direction, opposite to the first direction, to cause the mandrel         to rotate in the second direction relative to the outer housing         to move said at least one reaming element to the inwardly         retracted condition.

By providing at least one reaming element which is deployed outwardly by rotation of an inner mandrel relative to an outer housing, this provides the advantage that a user can apply rotation to the drill string via the rotary table or rotary top drive to enable reaming at selected times.

When used in a drill string conducting a slide drilling operation, the drill string is not rotated by the rotary table at the surface. However, since drilling mud is still being pumped, the mud motor rotates the drill bit at the bottom of the Bottom Hole Assembly (BHA) and drilling continues with the non-rotating drill string sliding down the wellbore. Consequently, in this configuration the reamer elements will sit in the inwardly retracted condition until the operator applies clockwise/right rotation to deploy the reamer elements. This provides a significant increase in control for the operator and eliminates drag on the drill string caused by reamer elements.

During a slide drilling operation, as the drill bit at the bottom of the mud motor turns clockwise, an anti-clockwise reaction torque is induced in the drill string. This must be absorbed to prevent the whole drill string from turning. As a consequence there is an induced left hand torque in the string which forces the reaming elements to retract as the outer housing is held rotationally static by drag on the wellbore. The apparatus is therefore automatically held in the unset condition in a slide drilling operation.

Alternatively, during conventional directional drilling, the clockwise rotation of the drill string will deploy the reamer elements. The reamer therefore enables both conventional reaming and retraction of the reamer elements to eliminate drag during slide mode. All of this is accomplished by rotation of the drill string alone without the need for any electronic, hydraulic or other control.

This also provides the advantage of a wellbore enlargement tool that operates in a drill string above a drilling (mud) motor. Known reamer tools ream the well bore to the gauge of the drill bit or less. These tools are often used to simply smooth the well bore and ream ledges created by drilling with downhole motors. However, these tools do not enlarge the well bore because the cutter blocks would contact the edges of the pilot hole created by the bit and would therefore prevent movement of the drill string along the wellbore.

The drill string is therefore advantageous as it enables both reaming the wellbore above a drilling motor and enlarging a wellbore above a drilling motor. In the case of reaming, known tools are generally fixed size tools with cutters on the outer diameter that are very close to the drill bit diameter in order to ream ledges and smooth the well bore. This can cause drag while sliding the drill string.

By providing reaming elements that can retract to a smaller size while slide drilling significantly reduces drag. When rotation is commenced the outer reaming element diameter can be larger than the drill bit which significantly improves the action of reaming ledges. On the other hand, while slide drilling the outer diameter of the reaming elements can be less than the drill bit to reduce drag. When rotation is commenced, the reaming elements can move out to facilitate enlargement of the well bore.

In a preferred embodiment, the downhole reamer apparatus further comprises:

-   -   at least one first recess formed in the mandrel, wherein said at         least one reaming element sits in said at least one first recess         in the inwardly retracted condition; and     -   at least one inclined surface disposed adjacent said at least         one first recess, such that when said mandrel is rotated         relative to said outer housing in said first direction, said at         least one inclined surface urges said at least one reaming         element into the outwardly deployed condition.

In a preferred embodiment, the downhole reamer apparatus further comprises at least one aperture formed in said outer housing through which said at least one reaming element projects in the outwardly deployed condition.

Said at least one reaming element may be retained in the apparatus by a pair of wings configured to engage an inner surface of said outer housing.

This has been found to provide the advantage of reducing vibration in the drill string.

In a preferred embodiment, the apparatus further comprises at least one retention spring arranged to retain said at least one reaming element in said at least one first recess.

This provides the advantage of aiding retraction of the reaming elements.

In a preferred embodiment, said mandrel further comprises at least one second recess in which at least one lug mounted to said outer housing is received, said at least one second recess being sized to limit the angle of rotation possible between the mandrel and the outer housing by engagement of said at least one lug with the sides of said second recess.

This provides the advantage of preventing the mandrel from bottoming out on the reaming elements in both the inwardly retracted and outwardly deployed conditions. The lugs engaging the second recesses in the mandrel take the force required to prevent unwanted rotation of the mandrel relative to the outer housing.

The apparatus may further comprise a torsional spring arranged to bias the mandrel to the position relative to the outer housing in which said at least one reaming element is in the inwardly retracted condition.

This provides the advantage of a permanent application of torque between the mandrel and outer housing to ensure that the tool is naturally biased to the position in which the reaming elements are retracted.

In a preferred embodiment, the drill string apparatus further comprises a plurality of reaming elements disposed around the mandrel.

The weight of the drill string may urge the mandrel to rotate in the second direction relative to the outer housing to move said at least one reaming element to the inwardly retracted condition.

It has been found that as well as the reaction torque produced by the drilling motor, in certain drill string configurations and wellbore conditions, the weight of the drill string causes rotation of the outer hosing relative to the mandrel to tend to urge the reaming elements into the retracted condition.

According to another aspect of the present invention, there is provided a downhole reamer apparatus comprising:

-   -   a mandrel arranged to be disposed in a drill string;     -   an outer housing disposed on the mandrel, wherein the mandrel is         rotatable relative to the outer housing; and     -   at least one reaming element moveable between an inwardly         retracted condition and an outwardly deployed condition in         response to rotation of the mandrel in a first direction         relative to the outer housing;     -   wherein said outer housing is moveable along a longitudinal axis         of said mandrel to enable locking means to be engaged to prevent         rotation of the mandrel relative to the outer housing.

This provides the advantage that the reaming elements can be locked in either the retracted/under gauge position or the outwardly deployed/over gauge position by use of the locking means. This is advantageous because it allows the reamer apparatus to control the direction of drilling of the drill string into which the downhole reamer apparatus is incorporated. This enables course corrections whilst rotating the drill string and therefore removes the requirement for slide drilling.

If the reaming elements are locked in the extended position, this causes the reamer to be full gauge in the wellbore and causes the drill bit to point downwardly slightly. When rotated, the string will therefore tend to drill downwardly as a consequence of the slight drop angle created.

Alternatively, if the reaming elements are locked in the retracted condition, this causes the reamer to be under gauge in the wellbore to cause the drill bit to point slightly upwardly. On rotation of the string, the resulting build angle tends to cause the string to drill upwardly.

During slide drilling, the rate of penetration falls off due to lack of lubrication and added friction. The reaming apparatus allows for up and down directional changes to be made during rotation. This greatly increases the speed of drilling and eliminates the requirement for slide drilling and the use of a bent sub.

In a preferred embodiment, said locking means comprises at least one first male spline or first female spline disposed on an end of said outer housing; and

-   -   at least one corresponding second female spline or second male         spline fixed relative to the mandrel;     -   wherein movement of said outer housing along the longitudinal         axis of the mandrel enables said at least one first male spline         or first female spline to engage the corresponding said at least         one second female spline or second male spline to prevent         rotation of the mandrel relative to the outer housing and set a         first outward extend to which said at least one reaming element         is deployed.

In a preferred embodiment, the apparatus further comprises at least one third female spline disposed on one of an end of said outer housing or fixed relative to the mandrel, wherein said at least one third female spline has a depth different to the depth of said at least one second female spline to prevent rotation of the outer housing on the mandrel when engaged with said at least one first or second male spline and set a second outward extent to which said at least one reaming element is deployed.

This provides the advantage of enabling the reaming elements to be locked at different extents of deployment to provide greater control for the operator.

In a preferred embodiment, the apparatus further comprises biasing means to urge the outer housing into a longitudinal position on the mandrel in which none of said first, second or third male or female splines are engaged to enable free rotation of said outer housing on said mandrel.

In a preferred embodiment, said outer housing is moveable along the longitudinal axis of said mandrel in response to an increase in fluid pressure in a longitudinal bore of said mandrel.

This provides the advantage of a simple control method for the operator to lock the reaming elements. When the drill bit is placed at the bottom of the wellbore and pumping from the surface commences, the pressure in the drill string increases. The operator can choose both a predetermined pressure and rotational state of the outer housing relative to the mandrel at which the outer housing moves along the mandrel to lock the reaming elements in the desired configuration. The operator therefore only has to control the pressure in the tool and the rotational state of the outer housing to set up the drill string for upward or downward drilling.

In a preferred embodiment, the apparatus further comprises at least one expansion chamber in fluid communication with said longitudinal bore, wherein an increase in fluid pressure in said expansion chamber causes said outer housing to move along the longitudinal axis of the mandrel.

In a preferred embodiment, the apparatus further comprises first valve means configured to prevent longitudinal movement of said outer housing on the mandrel until a predetermined fluid pressure is reached in said expansion chamber.

This provides the advantage of a reliable mechanism for enabling control over when the reaming elements are set.

In a preferred embodiment, the apparatus further comprises a hydraulic locking and return means defining a chamber of fixed volume, said chamber further comprising:

-   -   first chamber in which said biasing means is disposed;     -   a second chamber separated from said first chamber by said first         valve means, wherein a floating piston is disposed in said         second chamber such that when said first valve means opens when         fluid pressure in the expansion chamber reaches said         predetermined pressure, hydraulic fluid disposed in said first         chamber vents through said first valve means into said second         chamber to move said floating piston relative to said mandrel.

This provides the advantage of enabling the apparatus to be continually set and unset whilst downhole without having to be retrieved to the surface.

In a preferred embodiment, the apparatus further comprises second valve means disposed between said first and second chambers, said second valve means configured to enable hydraulic fluid to return from said second chamber into said first chamber in response to pressure in the expansion chamber decreasing below said predetermined pressure.

In a preferred embodiment, the apparatus further comprises at least one annular port formed in said mandrel to enable fluid to vent from the longitudinal bore into the mandrel when a corresponding port disposed on said outer housing is aligned with said at least one annular port.

This provides the advantage of enabling a user to monitor the downhole pressure in the longitudinal bore of the mandrel and therefore the configuration of the apparatus.

According to a further aspect of the present invention, there is provided a drill string comprising:

-   -   a drill bit;     -   a drilling motor, wherein the drill bit is configured to be         rotated either by rotation of the drill string or by operation         of the drilling motor; and     -   a downhole reamer apparatus as defined above.

This provides the advantage that the reaming elements can be locked in either the retracted/under gauge position or the outwardly deployed/over gauge position by use of the locking means. This is advantageous because it allows the reamer apparatus to control the direction of drilling of the drill string. Since the drill bit can be selectively controlled by either rotation of the drill string or by operation of the drilling motor actuated by pumping of drilling fluid, in the slide mode in which the drill string is held rotationally static but the drilling motor is operated, if the reaming elements are locked in the extended position, this provides a pivot point which causes the drill bit to point downwardly as a result of the weight of the drill string. This means that in the slide drilling mode, the string will tend to drill downwardly.

Alternatively, if the reaming elements are locked in the retracted condition, the weight of the drill string tends to point the drill it upwardly. This means that when slide drilling, the drill string will tend to move upwardly on drilling.

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a downhole reamer apparatus for use in a drill string of a first embodiment the present invention;

FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1;

FIG. 3 is a cross-sectional view of the apparatus of FIG. 1 showing the reamer element in the outwardly deployed condition;

FIG. 4 is a cross-sectional view taken along line B-B of FIG. 3;

FIG. 5 is a perspective view of mandrel of the apparatus of FIG. 1;

FIG. 6 is a perspective view of a downhole reamer apparatus for use in a drill string of a second embodiment of the present invention;

FIG. 7 is a cross-sectional view of the apparatus of FIG. 6 showing a reaming elements in the outwardly deployed condition;

FIG. 8 is a cross-sectional view corresponding to FIG. 7 but showing the reaming elements in the inwardly retracted condition;

FIG. 9 is a cross-sectional view taken along line E-E of FIG. 8;

FIG. 10 is a cross-sectional view taken along line F-F of FIG. 8;

FIG. 11 is a cross-sectional view taken along line C-C of FIG. 7;

FIG. 12 is a cross-section view taken along line D-D of FIG. 7;

FIG. 13 is a side view of a downhole reamer apparatus of a third embodiment of the present invention;

FIG. 14a is a cross-sectional view of the apparatus of FIG. 13 showing the reaming elements in the retracted condition;

FIG. 14b is a cross-sectional view of the apparatus of FIG. 13 showing the reaming elements in the extended position;

FIG. 15 is a perspective view of a reaming element of the apparatus of FIG. 13;

FIG. 16a is a side view of the reamer apparatus of FIG. 13 showing the reaming elements in the extended condition and the locking mechanism in a first locked condition;

FIG. 16b is a side view of the reamer apparatus of FIG. 13 showing the reaming elements in the retracted condition and the locking mechanism in a second locked configuration;

FIG. 16c is a side view of the reamer apparatus of FIG. 13 showing the locking mechanism in the unlocked condition and the outer housing freely rotatable relative to the mandrel;

FIG. 17a is a longitudinal cross-sectional view corresponding to the configuration of the reamer apparatus as shown in FIG. 16 a;

FIG. 17b is a longitudinal cross-sectional view showing the reamer apparatus in the configuration corresponding to FIG. 16 b;

FIG. 17c is a longitudinal cross-sectional view of the reamer apparatus in a configuration corresponding to FIG. 16 c;

FIG. 18 is a partially cut away perspective view of the reamer apparatus of FIG. 13 showing a swivel bearing;

FIG. 19 is a perspective view of the mandrel of the reamer apparatus of FIG. 13;

FIG. 20 is a perspective view of the cage of the apparatus of FIG. 13;

FIG. 21a is a side view of a drill string of an embodiment of the present invention shown downhole with the reamer elements in the retracted condition;

FIG. 21b is a close-up of the reamer elements of FIG. 21 a;

FIG. 22a is a view corresponding to FIG. 21a but with the reamer elements in the extended condition;

FIG. 22b is a close-up view of the extended reamer elements of FIG. 22 a;

FIG. 23a is a perspective view of a roller element comprising rollers;

FIG. 23b is a side view of the roller element of FIG. 23a ; and

FIG. 23c is a cross sectional view of the roller element of FIGS. 23a and 23 b.

Referring to FIGS. 1 to 5, a downhole reamer apparatus 2 for incorporation into a drill string (not shown) comprises a mandrel 4 arranged to be disposed in a drill string and an outer housing 6 disposed on the mandrel 4. The mandrel 4 is rotatable relative to the outer housing 6.

The apparatus 2 also comprises at least one reaming element 8 moveable between an inwardly retracted condition (FIGS. 1 and 2) and an outwardly deployed condition (FIGS. 3 and 4) in response to rotation of the mandrel 4 relative to the outer housing 6. Reaming elements 8 are retained in the apparatus by use of spline bars 22 projecting through apertures 24 formed in each reaming element 8. Spline bars 22 are received in grooves (not shown) formed in the outer housing 6. The spline bars 22 provide a strong and reliable means of retaining reaming elements 8 in the apparatus 2 whilst also preventing rotation of the reaming elements 8. Furthermore, by adjusting the width of spline bars 22, the operator can change the extent to which the reaming elements 8 deploy in a relatively straightforward manner.

Preferably, a plurality of reaming elements 8 are arranged around the mandrel 4 in an equiangular configuration. In the example shown, apparatus 2 comprises two sets of three reaming elements 8 disposed at 120° spacing around the mandrel 4. Each reaming element 8 sits in a corresponding first recess 10 formed in the mandrel 4 when in the inwardly retracted condition. An inclined surface 12 is arranged adjacent each first recess 10 to act as a ramp to urge reaming elements 8 to the outwardly deployed condition on clockwise rotation of the mandrel 4 relative to the outer housing 6 as shown from moving from the configuration of FIG. 2 to that of FIG. 4. Reaming elements 8 project through apertures 14 formed in the outer housing 6 and comprise cutting elements 20 to engage and enlarge the surface of a borehole.

The mandrel 4 sits inside outer housing 6 and is rotatable to the extent shown in FIG. 4 in which the reamer elements 8 are deployed. Inclined surfaces 12 are shown in the form of pockets formed in mandrel 4. However, the inclined surfaces 12 could also be formed longitudinally along the length of mandrel 4.

The outer housing 6 is connected at its end to a threaded connection 18. Mandrel 4 also comprises a female threaded connection part 16 such that the apparatus 2 forms a complete sub which can be incorporated in a drill string in a manner which will be familiar to persons skilled in the art.

The operation of downhole reaming apparatus 2 will now be described with reference to FIGS. 1 to 5.

Downhole reaming apparatus 2 is mounted in a drill string (not shown) in a manner which will be familiar to persons skilled in the art. During conventional directional drilling in which the drill string is rotated in the clockwise direction, when rotation commences, mandrel. 4 is rotated in the clockwise direction relative to outer housing 6. This moves mandrel 4 from the configuration of FIG. 2 to that of FIG. 4 and causes inclined surfaces 12 to drive the reamer elements 8 outwardly. Cutting elements 20 are therefore driven against the surface of the borehole to enlarge the borehole.

When the drill string operator intends to drill in slide mode, rotation of the mandrel 4 of the drill string by the rotary table at the surface is stopped. Fluid pumping is then commenced to rotate the bit (not shown) of the drill string by a drilling motor (not shown) in the drill string. A drilling motor (also known as a mud motor) generally uses a progressive cavity positive displacement pump operated by pressurised drilling fluid to rotate the bit, as will be familiar to persons skilled in the art. The drill string is now moved by being slid along the wellbore.

Rotation of the drill bit by the drilling motor causes an anti-clockwise (left hand) reaction torque. Due to the drag of the outer housing 6 on the sides of the wellbore, this causes the mandrel 4 to rotate anti-clockwise to the position of FIG. 2 in which the reaming elements 8 are retracted. The reaming elements 8 therefore do not engage the surface of the wellbore in this configuration to significantly reduce friction during sliding. Furthermore, the weight of the drill string assists in realigning the outer housing 6 to cause retraction of the reaming elements 8. It has been found that as well as the reaction torque produced by the drilling motor, in certain drill string configurations and wellbore conditions the weight of the drill string causes rotation of the outer hosing 6 relative to the mandrel 4 to tend to urge the reaming elements 8 into the retracted condition which is desirable.

It can therefore be seen that all the operator has to do to enable reaming is to release the drill string to allow clockwise rotation. At the desired time, the operator allows clockwise rotation (right hand torque) in the drill string which acts on mandrel 4 and rotates mandrel 4 in the clockwise direction relative to outer housing 6. This moves mandrel 4 from the configuration of FIG. 2 to that of FIG. 4 and causes inclined surfaces 12 to drive the reamer elements 8 outwardly. Cutting elements 12 are therefore driven against the surface of the borehole to enlarge the borehole.

To retract the reamer elements, the operator simply prevents clockwise rotation which moves mandrel 4 back to the configuration of FIG. 2 allowing the reamer elements to move back to the inwardly retracted condition. The weight of the drill string also assists in realigning the outer housing 6 to cause retraction of the reaming elements 8.

Referring to FIGS. 6 to 12, a downhole reamer apparatus for use in a drill string of a second embodiment of the present invention will now be described, with parts common to the embodiment of FIGS. 1 to 5 shown with like reference numerals, but increased by 100.

Referring to FIGS. 6 to 12, downhole reamer apparatus 102 comprises a mandrel 104 arranged to be incorporated into a drill string. An outer housing 106 is disposed on the mandrel such that the mandrel is rotatable inside outer housing 106. Reaming elements 108 in the form of larger block type reamer elements are disposed around the outer housing 106 and are held in the apparatus 102 by spline bars 122 projecting through apertures 124 formed through each reaming element 108.

Each reaming element 108 sits in a recess 110 formed in the mandrel. Each recess 110 comprises an inclined surface 112 and is biased outwardly by clockwise rotation of the mandrel 104 relative to outer housing 106 in the same manner as described in the first embodiment.

Referring to FIG. 9, which is a cross-section taken along line F-F of FIG. 8, and FIG. 11, which is a cross-section taken along line C-C of FIG. 7, the mandrel 104 also comprises second recesses 130 in which lugs 132 are slidably received. Lugs 132 are splined to the outer housing 106. The lugs 132 engaging with the sides of the second recesses 130 define the limits of rotation between mandrel 104 and outer housing 106. Lugs 132 therefore take the force of rotation to prevent the mandrel bottoming out on the reamer elements 108.

A torsion spring 136 is mounted between the mandrel 104 and outer housing 106 to bias the mandrel 104 into the condition of FIGS. 9 and 10 in which the reaming elements 108 are retracted. As a consequence, when no torque is applied to the apparatus 102, the reaming elements 108 are naturally retracted. On application of clockwise torque to the apparatus 102, the mandrel 104 is therefore rotated against the action of spring 136 to the extent shown in FIG. 11 in which lugs 132 engage sides of second recesses 130 and reaming elements 108 are deployed.

Although in the first and second embodiments, the method of retaining the reaming elements 8, 108 in the respective apparatuses 2, 102 is described by using spline bars projecting through apertures in the reaming elements, other means of retaining the reaming elements in the apparatus are envisaged. For example, the reaming elements could be formed as blocks with inclined grooves sliding in corresponding grooves formed in the aperture edges of the outer housing. A shoulder in the mandrel recess could then be used to retain the blocks in the outer housing. Alternatively, a pin disposed in the outer housing projecting into the block could be used.

Shear pins can also be used to prevent unwanted rotation of the mandrel and therefore unwanted reaming element deployment whilst the drill string is moved into a wellbore or, for example, when the drill string is being used to drill a shoe track. Furthermore, although the apparatus has been described wherein clockwise rotation of the mandrel deploys the pistons, the apparatus could be set up to rotate in the opposite anticlockwise direction to have the same effect.

Referring to FIGS. 13 to 17, a downhole reamer apparatus of a third embodiment of the present invention will now be described, with parts common to the embodiment of FIGS. 1 to 5 shown with like reference numerals, but increased by 200.

Downhole reamer apparatus 202 comprises a mandrel 204 arranged to be disposed in a drill string and an outer housing 206 disposed on a mandrel 204. The outer housing 206 and mandrel 204 are rotatable relative to one another to the extent shown in moving from FIG. 14a to FIG. 14b . Reaming elements 208 comprise wings 240 that bottom out on the inner side of outer housing 206 when the reaming elements are in the deployed condition. Deployment of the reaming elements 208 is therefore accomplished in substantially the same manner as the embodiments of FIGS. 1 to 12. The reaming elements comprise a plurality of cutters 242 to engage and ream the sides of a wellbore as the apparatus 202 is rotated. Retention springs 244 are provided to reduce vibration and assist retraction.

Referring to FIGS. 16 and 17, locking means 250 provides a mechanism for locking the reaming elements 208 at a particular extension relative to the mandrel 204. Outer housing 206 is moveable along the mandrel 204 to enable the locking means to engage and release as desired. The locking means comprises at least one first male spline 252 and at least one corresponding second female spline 254. An additional locking regime is provided by at least one third female spline 256. In the embodiments shown, the male splines 252 are fixed relative to the mandrel and the female splines 254 and 256 are mounted on the end of the outer housing 206 and rotate with the outer housing 206. As a consequence, the locking means 250 is engaged by the outer housing moving along the mandrel 204 to a position in which male splines 252 are interlocked with either second female splines 254 or third female splines 256.

Longitudinal bore 248 passes through mandrel 204 and receives pressurised drilling fluid pumped from the surface. Expansion chamber 258 is in fluid communication with the longitudinal bore 248 via internal port 260. As a consequence, when pressurised fluid is pumped from the surface into longitudinal bore 248, the fluid flows through port 260 into expansion chamber 258. However, movement of the outer housing 206 along mandrel 204 is prevented until such time as the fluid pressure in expansion chamber 258 reaches a predetermined pressure threshold. This is accomplished by the use of a closed hydraulic system forming a hydraulic locking and return means defining a first chamber 262 in which a biasing means such as a coil spring 264 is disposed, a second accumulation chamber 266 in which a floating piston 268 is disposed, a pressure release valve 270 and check valve 272 both disposed between the first and second chambers.

The pressure release valve 270 and check valve 272 allow fluid communication between the first and second chambers 262, 266 as will be further explained below. The first and second chambers 262, 266 in combination with the floating piston 268 generally define a chamber of fixed volume. Referring specifically to FIGS. 16c and 17c , in the neutral position of the outer housing 206 on mandrel 204, none of the male 252 or female 254, 256 splines are engaged and the outer housing 206 can therefore rotate on the mandrel. 204 to enable deployment and retraction of the reaming elements 208.

If the user wishes to lock the reaming elements 208 in the extended position, referring to FIGS. 16a and 17a , it is necessary to increase fluid pressure in longitudinal bore 248 and therefore expansion chamber 258. This is accomplished by placing the drill bit (not shown) of the drill string against the bottom of the wellbore and commencing fluid pumping to operate the drilling motor (not shown). This increases pressure in longitudinal bore 248 and therefore in expansion chamber 258. First valve means such as pressure relief valve 270 is set such that at a predetermined pressure, pressure relief valve 270 opens and permits hydraulic fluid to flow from the first chamber 262 through pressure relief valve 270 into accumulation chamber 266. This enables the expansion chamber 258 to increase in volume which causes compression of coil spring 264. At the same time, floating piston 268 moves upwardly to increase the volume of accumulation chamber 266 to accommodate fluid passing through pressure relief valve 270. The net result of this movement is that outer housing 208 moves along mandrel 204 to cause first male splines 252 to interlock with first female splines 254. This prevents further rotation of the outer housing 206 to lock the reaming elements 208 in the extended position.

Referring to FIGS. 16b and 17b , if the operator wants to retain the reaming elements 208 in the retracted condition, for example during slide drilling, the tool can be pressurised and the outer housing 206 moved to a position in which the first male splines 252 engage third female splines 256. In this condition, there is still some compression of coil spring 264 and therefore movement of fluid into accumulation chamber 266 to move floating piston 268. On reduction of fluid pressure in the body, check valve 272 opens and coil spring 264 expands to push outer housing 206 along mandrel 204 towards the neutral position of FIGS. 16c and 17c . Floating piston therefore moves to reduce the size of accumulation chamber 266 as fluid pressure equalises.

In order for operators on the surface to ascertain the configuration of the apparatus 202, a first annular port 274 is formed through mandrel 204 and corresponding openings 276 are formed in the outer housing 206. When annular port 274 is aligned with opening 276 as shown in FIG. 17a , fluid vents from longitudinal bore 248 to yield surface pressure readings at a known constant flow rate. This therefore indicates to the user that the reaming elements 208 are locked in the extended condition. Alternatively, the condition of the tool can be ascertained by torque. When rotating with the reaming elements 208 in the extended position, more torque will be required as the reaming elements 208 are engaging the formation.

Referring to FIG. 18, ball bearings 280 loaded in a bearing race 282 allow rotation of the outer housing on the mandrel 204. Referring to FIGS. 19 and 20, mandrel 204 is shaped to locate in positional cage 284.

The operation of the downhole reamer apparatus 202 in a drill string to enable directional drilling will be described with reference to FIGS. 21 and 22.

Referring to FIGS. 21a, 21h, 22a and 22b , drill string 312 comprises a drill bit 304 operable by both drilling motor 300 and by rotation of the drill string 312. Incorporated in the drill string 312 is downhole reamer apparatus 202 comprising a mandrel 204 rotatable relative to the outer housing 206. Reaming elements 208 are moveable between the inwardly retracted condition and outwardly deployed condition in response to rotation of the mandrel relative to the outer housing 206. As explained, operation of the drilling motor 300 to rotate the drill bit causes a reaction torque in the drill string to cause the mandrel to rotate and move the reaming elements 208 to the inwardly retracted condition. A stabiliser 302 is also provided in the drill string 312.

The drill string is located in wellbore 306 at the end of drill pipe 310. If the operator wishes to drill upwardly, the reaming elements 208 are locked in the retracted condition corresponding to the configuration of FIGS. 16b and 17b . In one example of the tool, this would give an outer tool diameter of 5 inches as shown in FIG. 21b . However, the drill bit has drilled the wellbore to a diameter of 6.125 inches. This means that the reamer apparatus is under gauge causing build angle 308 to point the drill bit upwardly.

Alternatively, to drill downwardly, the reaming elements are locked in the extended condition corresponding to the configuration of FIGS. 16a and 17a , which gives an effective outer diameter of 6.125 matching that of the wellbore. This causes a drop angle 310. The drill bit therefore points downwardly to enable downward drilling.

This enables directional changes to be made during rotary drilling mode when the whole drill string 312 is rotated to eliminate the requirement for slide drilling. This also enables directional changes to be made without use of a bent sub. This greatly improves the speed of drilling.

Referring to FIGS. 23a, 23h and 23c , alternative roller elements 312 can be used instead of the reaming elements. Roller elements 312 have rollers 314 which when deployed, can be used to roll out crimped or damaged wellbore casing.

Alternatively, reaming elements 208 can be provided with cutter blocks configured to mill casing. This would enable the reaming apparatus 202 to be used in section milling. Section milling is a method for cutting away large sections of steel casing in the well bore and circulating the steel cuttings out of the hole. Most section mills work using hydraulics and are designed to section mill with the work string moving downhole in compression. Reaming apparatus 202 would not require hydraulics and could be effective milling with the work string in tension instead of compression.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. 

1. A drill string comprising: a drill bit; a drilling motor, wherein the drill bit is configured to be rotated either by rotation of the drill string or by operation of the drilling motor; and a downhole reamer apparatus comprising: a mandrel arranged to be disposed in the drill string; an outer housing disposed on the mandrel, wherein the mandrel is rotatable relative to the outer housing; and at least one reaming element moveable between an inwardly retracted condition and an outwardly deployed condition in response to rotation of the mandrel in a first direction relative to the outer housing; wherein operation of the drilling motor to rotate the drill bit causes a reaction torque in the drill string in a second direction, opposite to the first direction, to cause the mandrel to rotate in the second direction relative to the outer housing to move said at least one reaming element to the inwardly retracted condition.
 2. A drill string apparatus according to claim 1, wherein the downhole reamer apparatus further comprises: at least one first recess formed in the mandrel, wherein said at least one reaming element sits in said at least one first recess in the inwardly retracted condition; and at least one inclined surface disposed adjacent said at least one first recess, such that when said mandrel is rotated relative to said outer housing in said first direction, said at least one inclined surface urges said at least one reaming element into the outwardly deployed condition.
 3. A drill string apparatus according to claim 2, wherein the downhole reamer apparatus further comprises at least one aperture formed in said outer housing through which said at least one reaming element projects in the outwardly deployed condition.
 4. A drill string apparatus according to claim 2, wherein said at least one reaming element is retained in the apparatus by a pair of wings configured to engage an inner surface of said outer housing.
 5. A drill string apparatus according to claim 1, further comprising at least one retention spring arranged to retain said at least one reaming element in said at least one first recess.
 6. A drill string apparatus according to claim 1, wherein said mandrel further comprises at least one second recess in which at least one lug mounted to said outer housing is received, said at least one second recess being sized to limit the angle of rotation possible between the mandrel and the outer housing by engagement of said at least one lug with the sides of said second recess.
 7. A drill string apparatus according to claim 6, further comprising a torsional spring arranged to bias the mandrel to the position relative to the outer housing in which said at least one reaming element is in the inwardly retracted condition.
 8. A drill string apparatus according to claim 1, further comprising a plurality of reaming elements disposed around the mandrel.
 9. A drill string apparatus according to claim 1, wherein the weight of the drill string urges the mandrel to rotate in the second direction relative to the outer housing to move said at least one reaming element to the inwardly retracted condition.
 10. A downhole reamer apparatus comprising: a mandrel arranged to be disposed in a drill string; an outer housing disposed on the mandrel, wherein the mandrel is rotatable relative to the outer housing; and at least one reaming element moveable between an inwardly retracted condition and an outwardly deployed condition in response to rotation of the mandrel in a first direction relative to the outer housing; wherein said outer housing is moveable along a longitudinal axis of said mandrel to enable locking means to be engaged to prevent rotation of the mandrel relative to the outer housing.
 11. An apparatus according to claim 10, wherein said locking means comprises at least one first male spline or first female spline disposed on an end of said outer housing; and at least one corresponding second female spline or second male spline fixed relative to the mandrel; wherein movement of said outer housing along the longitudinal axis of the mandrel enables said at least one first male spline or first female spline to engage the corresponding said at least one second female spline or second male spline to prevent rotation of the mandrel relative to the outer housing and set a first outward extend to which said at least one reaming element is deployed.
 12. An apparatus according to claim 11, further comprising at least one third female spline disposed on one of an end of said outer housing or fixed relative to the mandrel, wherein said at least one third female spline has a depth different to the depth of said at least one second female spline to prevent rotation of the outer housing on the mandrel when engaged with said at least one first or second male spline and set a second outward extent to which said at least one reaming element is deployed.
 13. An apparatus according to claim 11, further comprising biasing means to urge the outer housing into a longitudinal position on the mandrel in which none of said first, second or third male or female splines are engaged to enable free rotation of said outer housing on said mandrel.
 14. An apparatus according to claim 10, wherein said outer housing is moveable along the longitudinal axis of said mandrel in response to an increase in fluid pressure in a longitudinal bore of said mandrel.
 15. An apparatus according to claim 14, further comprising at least one expansion chamber in fluid communication with said longitudinal bore, wherein an increase in fluid pressure in said expansion chamber causes said outer housing to move along the longitudinal axis of the mandrel.
 16. An apparatus according to claim 15, further comprising first valve means configured to prevent longitudinal movement of said outer housing on the mandrel until a predetermined fluid pressure is reached in said expansion chamber.
 17. An apparatus according to claim 15, further comprising a hydraulic locking and return means defining a chamber of fixed volume, said chamber further comprising: a first chamber in which said biasing means is disposed; a second chamber separated from said first chamber by said first valve means, wherein a floating piston is disposed in said second chamber such that when said first valve means opens when fluid pressure in the expansion chamber reaches said predetermined pressure, hydraulic fluid disposed in said first chamber vents through said first valve means into said second chamber to move said floating piston relative to said mandrel.
 18. An apparatus according to claim 17, further comprising second valve means disposed between said first and second chambers, said second valve means configured to enable hydraulic fluid to return from said second chamber into said first chamber in response to pressure in the expansion chamber decreasing below said predetermined pressure.
 19. An apparatus according to claim 14, further comprising at least one annular port formed in said mandrel to enable fluid to vent from the longitudinal bore into the mandrel when a corresponding port disposed on said outer housing is aligned with said at least one annular port.
 20. A drill string comprising: a drill bit; a drilling motor, wherein the drill bit is configured to be rotated either by rotation of the drill string or by operation of the drilling motor; and a downhole reamer apparatus according to claim
 10. 